X-ray diffraction studies will be performed with the aim of understanding the physical basis of membrane stability. Biomembranes contain a very diverse set of chemically distinct lipid species which contribute to the collective behavior of the bilayer in different ways. It is known that the phospholipids spontaneously assemble into a variety of bilayer and nonlamellar structures. The aim of this research is to understand both the balance of colligative forces which stabilize bilayers in lieu of other structures and the way these forces arise from lipid chemical diversity. Specific aims of the research include: 1) A model has been developed which explains lipid phase instability in terms of the spontaneous curvature elasticity of lipid layers. This model will be experimentally and theoretically refined and tested. Experiments will be performed to understand the way lipid diversity effects elastic properties. 2) It has been hypothesized that variations in the spontaneous curvature of different bilayer compositions affect the interaction and biodistribution of lipiphilic agents (e.g., oils, alcohols, anaesthetics, oligopeptides, and many drugs). Toward this end, the partitioning and destabilization effects of lipophilic agents will be studied. 3) New techniques of measuring membrane elastic properties will be explored. 4). The hydration potentials of lipid headgroups will be investigated. 5) The phase behavior and structure of novel membrane lipids and systems will be studied.